In liquid chromatography, in particular high-performance liquid chromatography (HPLC), there is often the requirement for mixing, as homogeneously as possible, at least two different eluents according to a desired proportioning. This mixing ratio could also vary, in particular, over time. Typically, during the mixing of two eluents, two separate fluid flows that are each generated by a pump are fed to a T-piece by means of two tubular lines or capillaries. After the T-piece, there should be a homogeneous mixture of the two eluents corresponding to the specified mixture ratio.
During this mixing process, an error in the flow of one of the two pumps always leads to an error in the mixing ratio. For example, if reciprocating pumps are used, then typically after each stroke of each pump, a proportioning error occurs, so that a mixing wave appears along the axial course of the flow of the mixed eluent. In addition to this axial mixing wave, there is often also the problem that the two fluids are not uniformly mixed in the radial direction of the eluent flow, that is, a direction perpendicular to the direction of eluent flow. This applies at least for the part of the eluent mixture in the vicinity of the joining of the two eluent flows at the T-piece.
For generating an eluent flow that is mixed as uniformly as possible, it is known to use so-called active mixing chambers. A stirring element that is usually magnetically driven turns in an active mixing chamber, wherein an eluent composition is held constant within the mixing volume. The effect of a short-time pump error is thus reduced by a corresponding factor as a function of the mixer volume and is washed out with an exponential decrease at the outlet of the mixer.
In addition to active mixing chambers, so-called static, longitudinal mixers are known that are based on the principle of dividing the incoming overall flow into several parallel sub-flows, wherein the sub-flows cover flow paths of different lengths and are then joined together again. A mixing error that also exists in the individual sub-flows and is present over time and thus also in the axial flow direction therefore appears at the joining with correspondingly smaller amplitude and a corresponding time delay.
If the mixing ratio changes periodically, in particular for the use of reciprocating pistons, and if the volume of the longitudinal mixer is so large that the volume of a whole period can be held in the mixer, then mixer errors are averaged out in an especially effective way.
If the flow at the inlet of a longitudinal mixer is still unmixed (inhomogeneous) in the radial direction, then these radial inhomogeneities can negatively affect the effectiveness of the longitudinal mixer.
Therefore, it is known to connect a radial mixing device upstream from a longitudinal mixing device.
In practice, radial, passive mixing devices are used that are made from a tubular mixer line in which a flow-guiding or turbulence-forming element is inserted. Indeed, radial mixing would also be achieved by diffusion along a line for the fluid mixture, but this effect is so weak and the radial dimensions of the flow of the fluid mixture are so large that radial mixing just through diffusion would require too long a time span or too large an axial length of the flow of the fluid mixture. In contrast, the use of one or more flow-guiding or turbulence-forming elements in the line for the fluid mixture leads to a significantly quicker homogenization of the fluid mixture. The flow of the fluid mixture here could be generated in a turbulent or laminar way. As flow-guiding elements, in practice, in particular, so-called helix mixers are used. Such a mixing device is known, for example, from JP 2007 13 28 73 A. For this mixer that was developed for the mixing of one eluent with one test sample in liquid chromatography, a spiral-shaped or helical element is used in a tubular part. The helix has, however, only a few windings, wherein the diameter of this spiral-shaped or helical element corresponds essentially to the inner diameter of the affected line. The effect of this mixing device thus is that the flow of fluid to be mixed is set into a helical rotational motion, wherein the fluid is mixed, due to the helical rotational motion, after leaving the mixer element or flow-guiding or turbulence-forming element. However, the effectiveness of a mixer constructed in this way is, in part, insufficient.
In addition, static, radial mixing devices are known in which a helical, twisted tubular line is provided, with the fluid mixture to be mixed flowing through this line. The action of such a radial mixing device is that, in particular, the fast portion of the laminar flow within the helical tubular line, that is, the center (in the cross section of the flow), experiences a centrifugal force. This force drives this part of the flow out from the center in the direction of the tube inner wall onto the outer side of the helical tubular line. A flow must then flow back into the center from the more slowly flowing edge layers, in particular, the portions close to the tube inner wall onto the inner side of the helical profile of the tubular line, wherein a thorough radial mixture is then produced.